Processing of fluids is a large technical field finding applications in most industries. Processing of fluids flowing in a pipeline typically involves phase separation of the fluid contents and delivery of the separated constituents at a specified quality, according to subsequent use. For example the stream from a hydrocarbon well is separated into oil, gas and water, the phases being processed and cleaned for contaminants until a specification is met. The processing will typically involve injection of fluids such as chemicals, solvents or extraction fluids for enhancing the effect of the separation and processing equipment.
The most commonly injected fluids (admixtures) can be summarized as follows:
Scavengers/irreversible solvents (liquid for removal of sour constituents, such as e.g. H2S, Mercury, Mercaptans)
Corrosion inhibitors, Hydrate inhibitors, Scale inhibitors, Wax inhibitors
Drag reducers, Desalters, De-emulsifiers, Deoilers, Defoamers,
Antifoulants
Flocculants (enhancing the coalescence rate of the dispersed phase)
Condensate/hydrocarbon (extraction fluids)
Gas (flotation or alleviation of slugging)
Water (desalting or manipulation of the water cut of a multiphase flow mixture away from its critical value)
The respective admixture fluids are typically introduced into the flow of a pipe upstream of processing equipment, such as upstream of a separator. The flow can be any multiphase mixture of gas and one or more liquids, a single gas or a combination of gases, any liquid or mixture of miscible liquid components or immiscible components such as hydrocarbon liquid and water. Hence, the flow can be for example unprocessed well stream, produced water, processed oil-water flow, processed gas flow, produced water contaminated with dispersed and dissolved hydrocarbon, processed water flow contaminated with hydrocarbon liquid, or water subject to gas component removal (e.g. de-oxygenation). The range of surface tension, viscosity, pressure and temperature may vary considerably, and additional types of fluids or admixtures are also relevant.
Whereas the description above and below mainly relates to processing of hydrocarbons, the mixing of fluids is an essential unit operation also in other parts of the process industry, such as production of food (e.g. production of emulsion), pharmaceuticals, chemicals (reactive flow which may involve activators or reagents), paper (refining/treatment of pulp), melts (alloys) and other processes. These processes in general involve batch production using large vessels, where the different fluids are mixed by means of agitators. It is reason to believe that using pipe flow mixing instead of agitation in vessels is attractive both due to investment, operational costs, flexibility in production, safety and product quality.
Typically, the flow rate of the admixture injected into the pipe is extremely small as compared to the volumetric flow rate of for example a multiphase flow. The challenges with the feed of the admixture are therefore associated with obtaining a steady non-oscillating injection rate, safeguarding axial mixing and simultaneously achieving homogeneous dispersion and distribution of the admixture over the pipe cross-section of the multiphase flow in concern (radial mixing).
The resulting droplet size distribution of the dispersed injected admixture is affected by the mixer design, fluid properties and flow rates in concern.
Injection quills are the most common injection device for admixtures, but injection quills provide no effective distribution of the chemical into the multiphase flow. With requirement to achieve steady-state injection rate, also the turn-down in the flow rate of the admixture is limited. Nozzles normally provide better distribution than quills of the injected fluid into the continuous phase. Disadvantages are however associated with limitations in secondary break-up of droplets, narrow operational range of flow rate of the admixture (turn-down) and limited mechanical robustness. Also the scale-up towards higher pipe dimensions is questionable.
For the Sulzer mixer and similar static mixers the admixture is injected upstream of the mixer. The mixers are based on plates or baffles installed in series such that the multiphase flow is repeatedly exposed towards high shear forces in order to finally gain an acceptable mixture of the injected fluid and the continuous fluid phase. Typically this requires a considerable pressure drop (equivalent to high energy consumption, limitation in capacity or production rate) and long mixer installation units. Such mixers typically yield a fairly non-uniform droplet distribution of the injected admixture for practical lengths of the static mixers as only a part of the admixture is exposed to the high shear forces at the surface of the baffles or plates.
One-shot mixers such as chokes or venturies expose the inflowing multiphase flow towards a zone of high shear accomplished within a fairly short mixer. As for these mixers the injected fluid is pre-injected upstream of the mixer device, the injected fluid is entrained with the bulk of the continuous phase. Accordingly the injected fluid is in general not exposed to the part of the mixer where the shear forces are high; the vicinity of the mixer wall. In order to compensate for this and secure the break-up of the injected fluid (as associated with stretching of “fluid elements” in regions of high shear; large gradient in fluid velocity), a high pressure drop over the mixer needs to be imposed.
The Westfall Manufacturing Company of Bristol, R.I., USA offers a static mixer which is adapted for disposition in a pipe containing fluid flow, the static mixer including a circumferential flange radially inwardly extending from the internal pipe surface and in turn having at least a pair of opposed flaps extending there from and inclined in the direction of the fluid flow. Said static mixer is described in patent publication U.S. Pat. No. 5,839,828, to which document reference is made. Operation of the static mixer results in a combination of laminar and turbulent flow (column 1, line 36-39). Further, chemicals can be added through injection ports on the downstream side of the flaps (claim 4, FIGS. 10 and 7, column 3 line 21-33 and 59-62). In this device the chemical is point injected behind a plate, namely a flap, and not injected such that the chemical is homogeneously distributed in the continuous phase. There is no description of any sharp edge.
By the invention of the ProPure injection mixer designated C100, as described in patent application EP 01947618.3, the technology for mixing and injection was advanced. The injection mixer C100 consists of a contacting element formed as a contracting pipe through which a gas stream flows, and an injection element consisting of a liquid inlet configured to produce an annulus of liquid around the internal perimeter of the contracting pipe, a sharp edge at the end of the contracting pipe and a further pipe section downstream of the sharp edge. Preferably the downstream pipe section is a diverging pipe to recover some of the pressure dropped over the contracting section. In patent application EP 01947618.3 it is described how the injection mixer C100 can be used for distributing a liquid into a gas stream, for absorbing a selected gas component from a gas stream by bringing the gas stream into contact with a liquid including a solvent or a reagent for the selected gas component, for scavenging H2S from natural gas, for selectively removing H2S from a natural gas in preference to CO2, for simultaneously removing acid gas components from a natural gas stream, for deoxygenizing water, for dehydrating natural gas, and how it is used in combination with existing columns to adapt an existing plant to accommodate a change in the feed conditions. Additionally it is described how the injection mixer can be used as a mixer for remixing the phases in a fluid flow, without injection of chemicals. It is also described how several injection mixers can be combined in series or in parallel to inject several liquids, by injecting one chemical in each mixer (cf. claims 15, 16, FIGS. 10a and 10b of EP 01947618.3). Injection of several admixture fluids in one injection mixer is not considered in EP 01947618.3, probably because injection of several admixture fluids simultaneously is considered inefficient. For example, injection of a gas together with a viscous liquid is considered inefficient since the admixture fluids are not expected to mix intimately because of the large difference in fluid properties as density, surface tension and viscosity. This, combined with flow rate in concern and resulting pressure gradient over the injection conduit in concern, may serve to cause oscillating injection flow rates for at least one of the injected admixture fluids. Based on the teaching of EP 01947618.3 the person of ordinary skill in the art will only consider injection of liquid into a gas flow, only one injection element will be considered and only a contacting element formed as a contracting pipe will be considered, as there is no indications of different embodiments or the possibilities for improved technical effect.
Despite the advantageous properties of the C100 injection mixer, a demand exists for technology simplifying injection several chemicals or admixtures with one injection mixer, thus reducing the pressure drop and the number of injection mixers. A demand also exists for improved technical effect over the C100 injection mixer with respect to mixing of the admixture fluid, particularly with multi fluid injection, deposition of the admixture on the internal pipe wall, and also alternative constructions of an injection mixer, which can prove to be advantageous for specific applications, such as modifying existing equipment to improve the technical effect. A demand exists for an injection mixer with a steady, non-oscillating, minimized admixture injection rate (axial mixing) and homogeneous dispersion and distribution of the chemical into the fluid phases (radial mixing), over a wide range of flow conditions, with a narrow range of droplet/bubble sizes, at low pressure drop and low admixture deposition rate. A demand also exists for a mixer for homogenously mixing of fluids flowing in a pipe. A further demand exists for an assembly of an injection mixer with additional equipment, particularly feasible for treatment of produced water, treatment of oil, desalting and flow assurance.